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Could an electric engine provide the same performance as jet engines on current aircraft?
I have looked at various questions on this SE site regarding this topic but I have not really found a satisfactory answer. Some comments here regarding torque and power in electric motors vs gasoline engines were helpful. Additional info also found here.
Underlying my question is the assumption that battery technology will considerably advance from what is available commercially now. Simply assume that providing the electric motor with enough power is not an issue and that the weight of the battery is equal to fuel.
Is it possible to build an electric aircraft engine that would allow current commercial airplanes the same capabilities as jet engines available today do?

Assuming the question is about the engines only (and we are gifted infinite battery performance) - the only thing I can think of is electric engines can't do afterburner/reheat. However I believe very few commercial aircraft have ever offered this.

Thank you for your comment. Yes, I am asking only about engine (and plane) capabilities. I was wondering if there are any inherent characteristics that make electric motors unsuited for use as aircraft engines?

One problem is in the name: turbofans / turboprops rely on the turbo mechanic, i.e. fuel is burnt, exhaust used to drive a shaft, which in turn is ramming more air into the combustion chamber, allowing for higher pressures to build, increasing the power available. The whole concept won't work for electric engines, because there is no exhaust to work with and pressure is not (that) relevant for the efficiency of the engine. You'll probably be looking at "normal" propellers and limited speed for the time being, unless I've overlooked a way to drive a non-turbo above usual propeller speeds.

If you're using an electric motor, it wouldn't be a turbofan but rather a ducted fan. Modern jets have a turbine engine core with large bypass fans surrounding. The electric motor would replace the entire core of the turbine so you're left with a ducted fan.

@DevSolar I would suppose that a ducted fan wouldn't have the same speed limitation that turboprops experience. I'm thinking of something similar to high-bypass turbofans but with an electric core. I'm no expert but it seems similar to me.

Ill your battery weight be burnt in fly? Even if you pair each property of the engine/fuel system my actual airliner get lighter a few tons/hour

I submit that you can't simply handwave battery tech away. Li-ion batteries provide about 250 Wh/l and 350 Wh/Kg. Most petroleum distillates (from gasoline to jet fuel) are around 9000 Wh/l and 13,500 Wh/Kg. And there are good reasons (like its atomic weight and high half-reaction potential) why we're unlikely to do better than lithium.

The fundamental problem here is the laws of thermodynamics. Assuming you had a battery that could power the equivalent of a jet, it would cost more in fuel to charge the battery then to use jet fuel directly. This is the problem with electric cars today.

No, @user3344003, that's not the problem with electric cars. You're forgetting that an electric car is about five times more efficient at turning stored watt-hours into movement as an ICE car. So even if your charge/discharge round-trip efficiency is only 50% (and it's actually much better, around 80% or more) the electric car will still use fewer watt-hours. But in an airplane the battery mass is a much worse problem, because you have constantly spend energy to keep it airborne. Keeping the electric car (and its battery) rolling on the ground is not nearly as much of a load.

You still have to generate the electricity and transmit the electricity to the car. Compare the cost of heating a house with gas v. electricity.

"assume that providing the electric motor with enough power is not an issue" stop focusing on batteries and there associated weight. assume the electricity is coming from some soon to be invented super efficient lightweight nuclear reactor.

Not yet.
To look at a medium-range aircraft, let's base the engine on the CFM56 or the IAE V2500. Those engines produce between 100 and 150 kN static thrust. In cruise, their thrust is considerably lower due to the low density in cruise altitude and due to them moving at Mach 0.8. Let's use a value of 25 kN - this is enough so that two of them will comfortably push an A320 class airframe through the thin air at altitude.
The power to produce such an amount of thrust is force times speed. The speed when flying with Mach 0.8 in 35.000 ft is 240 m/s, so the power produced by one engine is 6.0 MW. Now let's look up how big and heavy an electric motor has to be to produce 6 MW continuously. As you can see from the linked Wikipedia page, the results are all over the place. Big, industrial motors come in at less than 1 kW/kg, so our motor would weigh more than 6 tons. The smaller motors for electric airplanes are pushing 10 kW/kg, the power-to-weight ratio of the GE90 turbofan, but will lose some of that when scaled up to size. Remember, even at 98% efficiency the motor will generate 120 kW of heat - this needs to be removed, and operating in thin air doesn't make this easy.
With current technology the motor could achieve maybe 2 to 3 kW/kg - this means the motor driving our hypothetical engine comes in at 2 to 3 tons. Add to this the fan and the fairing of the jet engine (we will not need the high-pressure part and all the turbines), but double the fan weight because we need to compensate for the missing high-energy core flow. This will maybe weigh 50% of the CFM56 / V2500, so we need to add another 1.2 tons.
The electric motor will be twice as heavy as the parts it replaces. There is still some work to do before it can gain an advantage over current jet engines, but it has some potential because it does not dump half of the energy supplied to it overboard in a hot, fast-moving, noisy gas stream.

EDIT:
Since so many people get excited about me omitting the energy density aspect of electric propulsion, even though the question did expressly desire to leave this out, here are two things to consider. Energy density is only half of the problem of electric storage.

The energy density of jet fuel is around 43 MJ/kg, while Lithium polymer batteries achieve not even one MJ/kg. But this comparison is linear thinking - realistically, the current will be produced either by a high-efficiency turbine-generator combination, or by fuel cells, burning hydrogen at twice the efficiency of a conventional jet engine. Since hydrogen packs 142 MJ per kilogram, at twice the efficiency the electric airliner would need only 162 kg of hydrogen for every ton of kerosene in a conventional jet. Yes, I know, even then its volume will still be a problem.
If any form of batteries is used, the fact that empty batteries weigh as much as full ones is the final nail in the coffin of battery-powered flight. While your average long-range jet lands at 60% of its take-off weight, the battery-powered jet would need to schlepp those heavy batteries all the way to the final destination. To be competitive, those hypothetical batteries would need to have twice the energy density of kerosene.

The engines you've mentioned are built for durability and to withstand some rather violent mishandling. Nobody asked them to be light :) So indeed there's still a road ahead to improve the weght/power ration :)

This analysis doesn't consider the weight of the power supply for the electric plane versus the weight of fuel for the jet.

@RussellBorogove: Please read the question. And understand it.

@GabrielVince: The Prius engine is 1.37 kW/kg. It is built for low weight - it is a car engine, not a stationary industry motor. I assumed 2 - 3 kW/kg, well knowing that weight is a premium in aircraft, but with current technology you will not get any better than that. After all, this motor needs to be certified just like any other component on the aircraft. Things would be different for Formula 1, but we are still looking at civil aviation here.

Thank you for your answer - exactly what I was looking for. One aspect I did not anticipate was the problem of dissipating the heat produced by the electric motor, very interesting.

@RussellBorogove from the question Simply assume that providing the electric motor with enough power is not an issue and that the weight of the battery is equal to fuel. No analysis of the weight of the power supply is necessary for this particular question.

@FreeMan Actually, the weight still is a problem even if you assume it's equal. See my answer. Though, of course, you're right that that analysis of what those weights would be isn't necessary, since it's assumed.

@lexeter I don't think that will be such a big problem. I ran the numbers, and the heat dissipation (measured in W/m²) would already less than your average CPU heat dissipation if your engine had only 1m² external surface area available. Although it is a problem for automotive applications, where a large supply of cold air will necessitate large, ugly scoops, this is arguably less of a problem for something that will basically be a giant desktop fan.

@sanchises Try covering a square meter in high-end CPUs running at full load and see how well air cooling works. :)

@reirab I specifically said 'average' CPU's (45W/269mm²). Secondly, try cooling that square meter with a 900km/h -50°C airflow - quite a difference from a CPU cooler (even a high-end).

@sanchises Ah, I missed the average part. Still, I'd wager you'll have trouble even with that kind of airflow, especially at the atmospheric pressure where airliners fly (1/3 to 1/4 of sea-level values.)

@sanchises Thank you for running the numbers! I am not doubting your calculation super powers but did you consider the reduced air density at altitude? Although I assume that while the thin air makes it difficult to dissipate heat it is also very cold which in turn should help. No idea, however, which of the two has a greater impact. Probably not easy to answer since it depends on many factors.

@lexeter Feel free to doubt my calculation powers ;-) And, I didn't calculate the actual engine temperature, only a order-of-magnitude. If you're really interested, the book 950+ page book I have on it (Foundations of Heat Transfer, F.Incropera et al) is a good start.

Turbofan engines manage to dissipate a whole lot of heat from the core. Not all of it goes out the exhaust. I don't know if it's equivalent

The hydrogen fuel cell energy density is puny. A hundred times that is available, but not used for other reasons.

"Remember, even at 98% efficiency the motor will generate 120 kW of heat - this needs to be removed, and operating in thin air doesn't make this easy." Yet a jet engine manages to remove much more heat in the same atmosphere. Perhaps you could attach some kind of electric fan onto the electric fan for cooling.... funnily enough 98% is at the range of the limit of the Carnot efficiency (assuming the air isn't liquid nitrogen and the engine is operating under the melting temperature of tungsten).

"an electric motor has to be to produce 6 MW continuously." Are you suggesting that jet engines run at 100% throttle during the entire flight?  Electric motors can handle a higher power intermittently (ie, during takeoff) than their rated power if they are designed to do so.  There may not be nice examples available commercially, but like jet engines an airplane electric motor would necessarily be customized to the application, and you should probably be looking for a motor somewhere closer to cruising power than maximum power.

Didn't know till today that "schlepp" also exists in english language :*)

Good comments on this subject, Peter.  Yeah the chief problem lies in the weight of the motors as well as the energy densities of current battery technology.  One idea might be the use of chemical fuels powering fuel cells and using the excess heat from the cells and the motor to increase the enthalpy of air from the ducted fan into the nozzle, creating additional reactive thrust.  This is just a concept and I haven't done a back of the envelope calculation to see if the combined weight of the motor, fuel cell, heat exchanger, etc would be lighter than existing jet engines.

@CarloFelicione: I agree, fuel cells could be an option. Their weight has come down quite a bit in the last 20 years, but they need to shrink by another order of magnitude to be ahead of all other options.

@AdamDavis Note that my 6MW number is already for cruise. At take-off, fuel flow and thrust are much higher. Yes, and I am suggesting that jet engines do run at 100% throttle, or close to it. They do this to be able to fly as high as possible in order to increase efficiency.

Hydrogen does not pack 142 MJ per kilogram.  That requires some magical container that will never exist.  Hydrogen requires extremely heavy pressure tanks and/or extremely heavy cryo systems with bulky/heavy insulation, and/or low pressure composite tanks that monopolize aircraft volume and have retention and icing problems. ...  There is a reason that, despite many attempts, hydrogen aircraft remain: rare, overpriced, and under-performing.

@BrockAdams As far as I know, the hydrogen in Zeppelins needed not much more than a lightweight bag of goldbeater's skin. And it still contained between 120 and 142 MJ per kg of potential energy. Please explain what is so magical about Zeppelins. They did exist!

@PeterKämpf, zepplins were not powered by hydrogen (they used gasoline or diesel). If they were, their range would only have been a few miles. (Actually much less as they'd almost immediately lose buoyancy and crash.)  On earth, you can't use hydrogen for propulsion at anywhere near 120 MJ/kg because of the equipment needed to store it.

@BrockAdams: Then this might surprise you. Please don't overlook the paragraph about Blaugas. This type of fuel contained hydrogen and was buoyancy-neutral.

@PeterKämpf, that zeppelin had an awful lot of petrol tanks for a "hydrogen powered" craft. Still Wikipedia says it could go 100 hrs on hydrogen alone (but it cites a source which says no such thing). ... Even if this was true, you'll note that the system performance is just a skosh below that of jet powered aircraft, per the question.

Any fuel based electric generation would defeat the point of an electric motor. You could just as easily feed hydrogen into an ICE or jet engine (or heaven forbid, rocket engine).

@Aron: Not necessarily. If you compare the efficiency of fuel cells plus electric motors with that of a gas turbine, the fuel cell wins hands down.

@PeterKämpf Let's assume batteries make up 75% of a plane's weight @ a future value of 500 Wh/kg = 1.8 MJ/kg. Then I calculate for the same energy storage, hydrogen + storage + fuel cells for the same airframe, etc. would take up ~10% of the plane's weight if they're 80% efficient. If induced drag is roughly half the drag on the airplane in the battery case, then the induced drag gets cut to ~10% of the battery case, but let's call it zero. So for fuel cells, you have half the drag of the battery case, and half of the power requirement.

@PeterKämpf Currently, electrolysis is about ~60% efficient, though. So you will need roughly the same energy that would be put onto the batteries in the plane, anyway. The cost difference per trip is close to zero. And that's not considering having to deal with the extremely pernicious hydrogen molecule, which you're either going to have as a cryogenic and costly to handle/store liquid or a gas with a volume larger than that of the batteries. Since the energy density increase of batteries seems inevitable, it mostly  has to do with the cost reduction on the battery side.

With current technology the motor could achieve maybe 2 to 3 kW/kg
Existing motors on propeller electric aircraft achieve 5kW/Kg, on Siemens Extra 330LE. High-temperature superconducting motors are demonstrated at ~8kW/kg, with good prospects of going about twice as high very soon.

@kert: Thank you for the information! I answered the question with current knowledge one year ago, and considering that the engine power is "five times more than comparable drive systems" (source) the answer is almost prescient. The more spectacular power/weight ratios are only possible with smaller engines; once you scale up to airliner scales, 8 or 10 are still impossible.

Actually, NASA has a design for 41kW/Kg at 12 megawatt scale. High temperature superconductors.
See https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150023038.pdf

@kert: Since you did not really read what you linked to: This is a projection using some heroic assumptions. Please note that I talked of the state of the art. The fine twisted MgB$_2$ simply isn't here yet, wishful thinking of NASA nonwithstanding. The question is about current aircraft, not NASA propaganda thought up to increase funding.

No i did read this, and other materials too. The same research team in collaboration with AFRL has produced working motors with HTS at 8kW/Kg before, its referenced in plenty of other research. Doubling that achievement is on near term horizon, and >40kW/Kg is modeled, with no fundamental constraints.

@PeterKämpf Fancy providing a calculation at a much higher altitude than current commercial flights? (15/20/30/50 km up?)

@FlorentHenry Greater height requires lower wing loading and bigger fan diameter. This will need more structure and cut into the payload fraction and should make electric propulsion look even worse. Granted, upwards of 20 km electric is becoming the only viable option since combustion engines will cease to work due to lack of sufficient partial pressure of oxygen.

@PeterKämpf different airplane shape (flying wing?), more fans, different materials could help get around the constraints of going higher up (which could require the air to be recycled and the plane to be sealed; a whole lot of different constraints for a different environment).

@FlorentHenry Believe me, a flying wing is not ideal for going high. If you want to know why, post a new question. Comments are poor for explaining controversial topics.

Engines are great as they are. Electric engines can be fast, powerful and effective. I see two problems:
First - the sheer amount of energy consumed by the commercial airplane. With a single engine giving out 200kN you need a small power-plant attached to the aircraft. Even batteries would be 100% effective and could store enough energy, you need to burn higher amount of fuel to charge them up (you would need a A LOT of alternative energy sources to match the energy given out).
Second - it is all about the energy density. Jet fuel having 34 MJ/l , batteries having up to 120 Wh/kg = 0.36MJ/kg (according to this site ). So you need more than 100x more space to store the same amount of energy.
Just search for the 'electric airplane' and you will get a list of mainly small, ultralight or self-propeled gliders where they don't need to carry a LOT of energy with it.

You've hit the nail on the head @GabrielVince, it's all about energy density. the best battery technology in current development has about 5-7 times a lithium ion battery, but that's still puny in comparison to hydrocarbon fuels. Even explosives don't match them for density - C4 only has 6.3 MJ/kg!

Thank you for your answer. As mentioned in my question I am aware of the battery technology issues. I was wondering if there are any inherent characteristics that make electric motors unsuited for use as aircraft engines. The first paragraph of your answer indicates that this is not the case and we could build electric motors with the same capabilities as turbofans - the only (admittedly huge) problem is supplying those electric motors with enough energy.

Electric motors do not have to get all their energy from batteries. One could put a small nuclear power plant on board (perhaps not a great idea if it's fission, but if one assumes at some point small scale fusion plants are a possibility ...). Before you deride this too much, remember people said the same about nuclear powered ships / submarines.

@abligh Current nuclear-powered ships are owned and operated by militaries. They have the knowledge to handle fissile material appropriately and, perhaps more importantly, the weapons to defend it. As far as fusion is concerned, there are no current fusion reactors that produce net energy output, small-scale or otherwise. Fusion power has been "a decade or two away" since at least the 1940s.

@reirab I agree. But OP does not limit his question to civilian jets. Also OP is assuming battery technology advances hugely, so it is not unreasonable to assume fusion technology advances hugely in the same time. I think the arguments re engine weight are more pertinent than those about power sources. Arguably a nuclear powered plane would be more efficient with a turbine based system than going to electricity and back.

"One could put a small nuclear power plant on board" -- if you've got a nuclear power plant, you can use the heat from that directly: https://en.wikipedia.org/wiki/Nuclear-powered_aircraft

@GdD actually, as a general rule, explosives are not very good stores of energy at all.  They make their name not by storing a lot of energy, but by their ability to let all of that energy lose in a remarkably short period of time.

@abligh Are you saying your engineering chops are better than both of the two most advanced militaries in the world? Because neither managed to make it work.

@Aron no, I did not say I could get it to work. But the suggestion scientists could do something in the next 20 years they failed to do 60 years ago seems reasonable.

@abligh No. It really isn't. The problem space is simple. Nuclear reactors are radioactive. Humans and radioactivity do not mix. Shielding is heavy (scientific fact, they need to be to have a large cross-section). Heavy planes don't fly [citation needed]. Don't get me started with neutron radiation and fusion.

And - do not forget the planes crash - a few every year :) You don't want to have a crashed and malfunctioning nuclear powerplant in your garden

@abligh also people tend to get nervous when you build nuclear reactors where a massive fireball could appear (see Chernobyl, Fukushima and Windscale).

There is one important disadvantage that batteries will always have compared with fuel combustion for aviation propulsion: the weight remains constant. Airliners (especially those used for long-haul flights) burn off a large percentage of their take-off mass over the course of the flight. Batteries, however, retain their initial mass constantly. This is a problem for a number of reasons:

The most obvious reason this is a problem is that more energy is required for the flight. Even if you get a battery that has equal energy density to jet fuel and is also stable (which we are currently quite far from,) the airplane will need to carry the entire mass of the batteries for the entire duration of the flight. Thus, as the flight goes on, far more energy will be used per mile on a battery-powered flight than on a fuel-powered one, even if the batteries have the same energy density as the fuel. This also means that even more battery mass will be needed for the same range, since that extra energy requirement has to come from the batteries.
The other big problem is maximum landing weight. Many airliners are not designed to be able to land at their maximum takeoff weight simply because it isn't needed. This is one of the reasons why fuel sometimes needs to be dumped or burnt off before an aircraft that encounters a problem after takeoff can land again. However, with batteries, you'll still be at take-off weight when you're landing, which means you'll need stronger landing gear and tires, which means yet more weight and design/manufacturing cost. It also means you'll be landing faster (because of the extra weight,) so you'll need more runway length to land as well as needing brakes that can absorb more energy. The kinetic energy of the airplane is equal to half its mass times its speed squared, so the energy that must be absorbed by the brakes during landing increases dramatically as landing weight and speed increases.
A somewhat less important, but still significant problem is more punishment to runway surfaces. With planes now landing near their MTOW, runways surfaces will be damaged more quickly and will need to be resurfaced more often and/or designed for larger loads than they are now. This would also probably mean that the aircraft wouldn't be able to access as many runways as an otherwise-equivalent fuel-powered aircraft would be able to use until those runways were strengthened.

Of course, you could start jettisoning battery cells as they're depleted, but this also (obviously) has a lot of problems:

In order to deplete some batter cells sooner than others, you won't be able to draw on all cells in parallel, which will mean higher power draw levels per cell (and, thus, more heat produced per unit time per active cell, etc.)
You'll need to design the airplane to be able to jettison the cells safely. This is doable, but will require lots of extra cost in design effort and extra weight.
Environmentalists won't be too happy when you start dropping huge batteries all over the place. Neither will property owners. Existing battery chemistries are already quite corrosive and a battery chemistry with the energy density of Jet-A is probably going to be even more corrosive, unstable, and otherwise bad for whatever it's dropped on.

Thank you. I was aware of 1 and 2 but not 3. Adds an interesting perspective; I have no idea what the costs for a resurfacing or strengthened runway design could be. However, I think if our civilization started using electric passenger jets the incentive to develop stronger runways would be big enough so that someone would come up with a solution for that particular problem (e.g., cheap new super-strong asphalt formula).

@lexeter We can build stronger runways now, it's just more expensive and doesn't happen overnight. Airports would have to have significant demand for it before they'd make the investment, especially in the case of airports where shutting a runway down to resurface causes major traffic headaches during the construction.

What about fuel cells? :)

@sanchises The question was asking specifically about batteries. Also, why would you use an electric fan with fuel cells instead of just burning the fuel directly in the engine to start with? The latter is almost certainly more efficient, not to mention less complex to design and maintain.

@reirab, a fuel cell approaches 100% efficiency at turning chemical energy into electricity.  Combustion is at best around 40% efficient.

@Mark Hmm... While Wiki is always to be taken with a grain of salt, it claims 40-60% is typical before considering losses in converting the electricity into propulsion. Also, like I mentioned earlier, this isn't really relevant, since the question is about batteries, not fuel cells.

The biggest advantage of 'going' electric is that electric fans are way more efficient than a jet turbofan. A jet turbofan creates 75-85% of its thrust from the fan and 25-15% from the 'core' exhaust stream. The principle is that slower the accelerated air is, the more efficiently you generate thrust, as moving a small volume of air very fast means you lose energy in the kinetic energy of the accelerated air mass. So, bigger (or more) fans, accelerating a larger volume of air at a slower speed is much more efficient. Jet engines already do this by connecting a large fan at the front to the compressor shaft behind it, and this is a high bypass jet engine.
Even so, modern turbofans achieve under 2 Newtons of thrust per kW of energy. This is because the engine itself has low thermodynamic efficiency coupled with the fan being sub-optimised by various constraints that do not apply to an electric fan design. For example, the fans diameter is limited by ground clearance and by the RPM of the compressor drive shaft. It still rotates much too quickly and the tip velocity is capable of going supersonic. This makes for drastic drag loses and noise issues. Consequently the bypass ratio is far too low for really high efficiency, which can only really be resolved by having multiple fans. having additional contra-rotating open blades electric fans for example around the rear of the fusilage can ingest slow air from the body of the aircraft which is more efficient, and they can be placed at multiple points along the wing and tail sections.
Electric fans can, due to the approx 4 times less thermodynamic energy loses and slower tip speed, optimal RPM and slower air exit velocity potentially exceed 20N per kW, and probably get to 35N per kW.At high speeds though I don't know what performance they would achieve but it is safe to say it is going to be a LOT better than a turbofan. Consequently a battery can potentially be competitive at around 500Wh/kg, including power electronics and wiring.
Motor weights depend on power required, since as pointed out its harder to cool a large core. However you wouldn't want to try to replace the fan on a current jet engine but have multiple lower power fans, which means that the power density in kW/kg is going to be higher than in the cases listed above, superconductors not withstanding. Smaller fans also can rotate faster, suiting these kind of motors.
As pointed out above, the REAL issue is not the battery energy density but the battery POWER density - to have not only sufficient power at take-off but also to recharge within a turn around of 20-50 minutes. Since electric aircraft would first be competitive only against short to medium haul flights, a lot of which are internal and have fast turn around times, a power density of around 1kW per kg is needed, and that exceeds the current capability of high energy density batteries by a large margin.
Theoretically, if we can gain greater thrust efficiency (say 60 Newtons per kW) then we could use a lot less energy, therefore we would only need to cycle a portion of the battery capacity and could get away with, say 500W per kg charge/discharge power). In practice the power density stated is a maximum value but occurs at lower energy efficiency and tends to shorten battery life, so the battery would need to probably have a stated power density 50% more to operate efficiently this way.
110 Newtons of thrust per kW of power has been demonstrated with electrostatic ion thrusters (the type used in 'lifters' which you can see on youtube) but this have low thrust density so you have to factor in weight. Increasing the voltage will help with that.
The issue of the plane not becoming lighter as you fly is to some extent important, but the cost saving in fuel and the potential to have many fans utilised for example to assist in airflow around the wing can increase lift at low speed and thereby compensate for increased mass through out the flight envelope. The likely embodiment of contra-rotating propellers each with electrically operated variable pitch blades that can optimise both tip velocity and angle to the conditions along with accelerating a much larger volume of air more slowly will greatly increase the overall efficiency. Electric propulsion in contra-rotating fans is much easier mechanically than one hooked to a diesel or jet turbine, and can suit the high velocity of commercial aircraft (see https://en.wikipedia.org/wiki/Propfan) which shows that contra-rotating ductless fans can offer efficiency advantages. Noise issues are a function of having to link these propellers to individual jet engines which again means high tip velocity as a product of the limitations on diameter and high RPM of the engine. When electrically powered, more fans can be used at much slower tip velocity, this slashes the noise produced.
The weight of the extra fans is partially offset by the gains from taking away the cowling both in weight and drag.
Because of the recharge issue, the likely embodiment will be advanced, higher efficiency engines that recharge the batteries once in cruise and descent, and top-up the energy required during climb. These could use superconducting generators and with an adequate battery reserve risk of catastrophic generator failures should be mitigated.

You can reap the increased efficiency of "electric fans" primarily by flying more slowly. Then do away with the shroud and you end up with regular propellers. Then it all makes sense.

Most batteries are able to fully recharge in less than 1 hour. Many in 30 minutes. 10 minutes to 80% charged is not out of question with adequate thermal management. I don't see this changing with increased energy density, so I don't see why power density would be an issue. But yeah, as Peter said, I would be interested in your thrust per kW at mach 0.85 speeds.

(While this question has long been answered, I feel there's a bit that can be added as it keeps getting asked and the technology is not entirely static.)
Let's first look at the power/weight ratios. The highest number for Tesla's car motors is 8.5 kW/kg. The electric-aircraft-specific Emrax 268 delivers about 11.6 kW/kg.
In comparison, the Trent XWB delivers 430 kN of thrust at 300m/s flow rate, which equates to 64.5 MW of power, in a 7,550 kg package - a power/weight ratio of 8.5 kW/kg. However this isn't apples to oranges: this ratio is for the whole package, engine and fan, and measures useful output, like wheel power for a car.
In short, turbine engines are still lighter than electric motors, but the difference is not dramatic.
Where all-electric powerplants fail to stack up is range. I've elaborated on it in response to another question - Are there any hybrid electric planes?. The short version is that the maximum possible range of an electric aircraft is 10 nmi for every % of its weight dedicated to the battery. This limits the range of electric aircraft to 300-450 nmi, if sticking with the fuel fractions of known airliners.
But there are aviation applications where this is enough. The most important non-renewable resource consumed by modern aviation - the supply of 1960s Cessnas and Pipers, without which no one could afford to become a pilot - will not last forever. Should authorities permit it, mass-produced Tesla powerplants could power trainers and GA planes at a fraction of the ownership cost of a certified avgas engine.

All comments very true and valid. I would only like to add that Siemens has made and flight tested a 260kW aeroplane engine with a 5kW/kg power to weight ratio specifically for the single engine ICE type of aircraft and believes the design is scalable such that 100 seater regional series hybrids could be a reality soon. Important to note here is that hybrids address battery energy density issues as well as take-off vs landing weight and the electric motors dramatically improve the safety over conventional ICE's (internal combustion engines)

Hi, and welcome to Aviation.SE.  Thanks for a good first answer. Adding references to this answer would make it even better.

Here's the link to Siemens' website regarding the mentioned aeroplane engine and its use in an Extra 330LE aerobatic airplane.
http://www.siemens.com/press/en/feature/2015/corporate/2015-03-electromotor.php?content[]=Corp

Roughly, potentially, but there are some key differences in the comparison of a jet engine and the theoretical 'electric jet engine', that are very different from the comparison of a car engine to a motor-driven EV.
Most notably, as previously mentioned, is the turbo-fan is mechanically driven by the combustion heat-driven expansion of the air compressed by it's compressor. At cruising speeds (where the jet engine is optimized), this is a much more fuel efficient arrangement than the cruising speed operation of an automobile combustion engine.
Basically, there are two places where the released heat is converted to mechanical energy-- first, much of the heat-release of combustion is captured by the turbine that drives the compressor. Second, the exhaust nozzle also converts heat not captured by the turbine into kinetic energy by accelerating the mass-flow through the engine, converting a pressure delta generated by heat expansion into a velocity delta through nozzle geometry. By comparison, the combustion engine converts the exhaust gas heat expansion into mechanical energy by driving a linear piston, and gains no mechanical energy by exhaust. Generally, turbines are more efficient than pistons at mechanical energy conversion. There's also a tertiary efficiency-- namely that combustion at high pressures more efficiently converts heat to pressure as the gaseous density is higher, so more of the chemical energy of the fuel is converted to kinetic energy in a jet engine than a combustion engine, simply by virtue of the higher pressure of the combustion reaction in the jet engine. The 'downside' for the jet engine is that to make the whole arrangement work fuel efficiently you have to be operating at a significant fraction of Mach, much faster than ground transport can manage safely. Hence, combustion engines rule the earth and jet engines rule the sky in the current paradigm.
So, even assuming unlimited power supply, you would still have to have to have a very efficient motor on an energy cost-efficiency basis. To boot, you would have to have an engine that operated at similar cruise velocities. Even leaving aside infinite power generation, we can still consider that more time in the air is a longer time-frame along which the aircraft must be energy self-sufficient, generally equating to more mass in battery and/or power generation. More mass lowers the mechanical efficiency on an aircraft operation basis, because it is more energy you have to spend to accelerate and decelerate the extra mass.
So in an electric motor driven equivalent, you probably still have something resembling a turbo-fan. Except that your motor is primarily driving your compressor fan, and the turbine is there mostly to recapture some of the energy of compression (which also generates heat) into energy to drive certain engine functions like coolant and lubrication circulation, possibly some power regeneration. So probably a smaller turbine, but this puts you up against the inconvenient fact that compressing air is not very energy efficient as a means of generating thrust. If it were, we'd be running aircraft off compressed air.
What this generically gets at is that the electrification of air travel is likely not to resemble current jet-era technology. It's within the bounds of known technology to apply efficiency of electric motors to the problem of air transport, but the resulting architecture is likely to be very different, much as the fundamental architecture of a full EV is different than a gas automobile. This will likely in addition mean some fundamentally different infrastructure.
E.g. much of the energy of a flight is taken up in the initial acceleration, so it's possible that an Aerial EV would take off from a runway that more resembles that of an aircraft carrier than a flat road, with an assisted launch. Similarly, recapturing the energy on landing could again utilize a system more similar to those seen on aircraft carriers, only dedicated to regenerative capture rather than rapid deceleration.
More directly, though, the fundamental problem is generating thrust at near-Mach speeds. The efficiency of electrical motors at turning electrical power into rotational mechanical power is somewhat mitigated by subsonic and supersonic fluid mechanics, because an aircraft has to generate thrust by accelerating an airflow, or 'pushing' against air in some way or another. At these speeds, propellers basically start to lose their efficiency, and propulsion methods above these speeds therefore rely on the expansion of gasses with the transfer of heat into the gas. So to compete in these speed areas, an energy efficient means of transferring heat to (compressed) airflow has to be devised, which is very different than simply applying known electric motor technology.

Except that your motor is primarily driving your compressor fan but that's already what happens, the turbine is there just to extract the energy needed for the compressor and fan.

much of the energy of a flight is taken up in the initial acceleration  - only for very short range, low level flights. For any realistic condition your assumption is plain wrong and belongs in a field called ballistics.

@ Fedrico What I'm getting at is that whereas a petro-fuel jet engine is driven by it's turbine, converting heat release from fuel combustion into rotational energy to drive the compressor, an electric motor driven subsonic/hypersonic 'engine' would apply the electric motor to the compressor fan, as the the compressor would be the most mechanically efficient place to apply rotational energy driven by an electric motor. In this arrangement, it still may be energy efficient to recapture some of the energy with a turbine, but the turbine is not driving the engine, the electric motor is.

@Peterkampf, the Airbus E-Fan already applies an electric motor on it's main wheel to accelerate during takeoff, getting the aircraft to around 60 KPH.
A ground launch assist would achieve similar effect, but reduce mass on the flight vehicle-- that launch-assist motor on the E-fan is dead weight for the rest of the flight. With ground launch assist, you might save more energy, and extend range, of an aerial EV. Particularly if the assisted launch provided 100% of energy for takeoff velocity. The Navy uses assisted launch for short takeoff, no reason it can't be used for energy efficiency.

@JasonHubbard: An aircraft with 20% fuel fraction and 25% efficiency (conversion of chemical energy into work) uses 0.0346% of the energy needed for the trip to accelerate to a take-off speed of 150 kts. On a comparable electrical aircraft you will be able to save 0.0346% of the hypothetical battery capacity by using a catapult launch, or you can stretch its range by a similar fraction. You will gain far more efficiency by e.g. choosing favorable winds enroute, or spending the catapult expense on a lighter structure.

Well the thing we need to start consideration is that jet engines are capable of providing a very high amount of thrust, yet they function on a simple principle of a gas turbine. Somehow it is possible to make an electric engine for propulsion but it will be complex and very heavy and lower powered. The only way an electric engine would work is to replace the jet engine core with some electric motor sort of, that can rotate the fan disk, creating thrust; however, considering how much torque is required to spin it in order to generate reasonable thrust is a nightmare, also the motor needs heavy batteries.

Yes.  What your question boils down to is, essentially:

Ignoring power input, can an electrical engine produce equivalent output to a jet engine within the size and weight of that jet engine?

So:

Is the power to weight ratio of a jet engine greater than electric engines?

and

Is the power to volume ratio of a jet engine greater than electric engines?

https://en.wikipedia.org/wiki/Power-to-weight_ratio#Electric_motors.2FElectromotive_generators
The GE90-115B Brayton turbofan jet engine used on the Boeing 777 has a power to weight ratio of 10.0 kW/kg.
An electric motor made for aviation, the EMRAX268, has significantly lower power, but attains 10.0 5kW/kg.
Some will be concerned about whether the motors can scale up but as can be seen in the electric vehicle industry fast electric cars are readily available, and the size and volume of of just the motor and its required components (cooling, control) is smaller and lighter than the gas engines for those vehicles that can compete in terms of acceleration and top speed.
Even more telling is that the electric motor and its components are not only lighter and smaller, but they are cheaper as well.
The only limiting factor to electric aviation is the power source, and as a step forward every major manufacturer is already designing hybrid electric planes.  As powerful as jet engines are, they are still not as fuel efficient (and thus emissions efficient) as they could be.  A jet fuel powered generators powering electric engines may be available in the market within a decade.
These hybrid planes wouldn't be possible if the electric engines couldn't hold their own in size, weight, and power relative to a jet engine.

Yes, they could.  As you said, assuming the power SUPPLY issue has been solved.  At it's heart, a jet engine heats air, harnesses the expansion to drive a compressor and in most turbofans drive the "propeller".  While presently all jet engines use burning fuel to produce that heat, the underlying principle of the system shouldn't care where the heat comes from.  If you could dump enough energy through electric heaters in the combustion section of an otherwise standard engine, I think you could in theory operate the exact same engine off of electricity.
For a modern turbofan, that'd be about 35MW of power you'd have to dump into air heaters in the "combustion" section.  This would be a pretty big engineering challenge, but I don't think it's out of the realm of possibility in theory.  One option might be using plasma sparks, like an arc welder.  Again, electrode lifespan would be an issue, but not necessarily impossible.  Credit for this idea comes from this page:
http://contest.techbriefs.com/2013/entries/aerospace-and-defense/3129

Battery and motor technologies aside, there is one major problem with the application of electric engines on commercial aircraft and that is recharge time. Commercial aircraft only make money when they're loaded with revenue paying passengers and in the air; when they're on the ground, it's extremely costly.   Conventional chemical fuels not only pack a great deal of energy density but are also extremely easy  to replenish the supply of. Refueling a passenger plane takes a matter of minutes - in some cases as long as a half hour for say an A-380 or a 747.   Batteries currently takes hours to recharge so this would have a huge implication on the delay a passenger or cargo flights.
Personally, I  don't see an electric powered commercial aircraft as a viable alternative to current jet engines. Probably the cleanest form of air travel would be current high-efficiency jet engines powered by a carbon neutral bio diesel fuel.

The question is about engines, not batteries or fuel. Anyway, most lithium batteries accept over 1C charging current w/o significative damage, given good thermal management. That translates in 80% charge in about 30 minutes. Just look at electric cars (tesla, leaf, etc) or smartphones with quick charging.

Another solution would also be to replace (part of) the battery back, which could be easily done within minutes. However, as pointed out, the question was about engines assuming power supply is not a problem.

Then you have to figure out where to fit said battery packs in the aircraft.  It would be difficult it they were part of the wing structure.  And fuselage space is already consumed by passengers and cargo.


text:
Does the dihedral effect happen during coordinated flight?
I have researched some about dihedral effect due to a question that I had, and I discovered that my question is essentially already answered, here and here, collectively.
This has led me to a new question because the information talks about slips.
If an airplane is flown in perfect coordination during a turn, does the dihedral effect still work to level the wings?

Related: How does the dihedral angle work?. It shows why the flow must be asymmetrical for the dihedral angle to work.

No.
The dihedral effect depends on a flow asymmetry between left and right wing. Coordinated flight means that both wings have the same flow conditions, and the only asymmetry between both is caused by control deflections.
You do have a flow asymmetry during a turn (coordinated or not) from the speed difference between the inner and the outer wing which grows with the turn rate and wing span. This does indeed cause more lift on the outer wing, but due to the difference in dynamic pressure, not due to dihedral. Since dihedral causes a side force component of lift, you can argue that the outer wing creates more side force which pushes the plane into the turn. But the pilot will compensate the difference in dynamic pressure with the ailerons to stop any rolling motion once the desired turn rate is stabilized. This will also equal the lift on both wings.
Another asymmetry is caused by the location of the vertical tail aft of the center of gravity. Essentially, the tail swings around during the turn, which causes a sideslip condition at the vertical tail. The result is a side force which grows with turn rate and tail lever arm, and due to the high location of most vertical tails also causes a small rolling moment. But again the wing's dihedral has nothing to do with that.
Dihedral only comes into play if you build up a sideslip, and then the turn is not coordinated any longer. @Zeus is right with his point that dihedral stabilizes the aircraft and makes it easier to keep the turn a coordinated one. Now we can split hairs all day if flying in turbulence makes a perfectly coordinated turn impossible. When it comes to dynamic stability, a moderate dihedral effect is highly recommended.

No but yes.
In a perfect turn (or level flight), everything will be symmetrical (except for the minor effects that Peter mentioned). So seemingly no effect.
However, the whole concept of stability requires that when a disturbance occurs, an opposing force or moment is generated. Even if you think that you are making a perfect turn, this perfection comes largely from inherent stability of the airplane, which automatically compensates small disturbances that are always happening. This includes the 'dihedral effect'.
Try to make a perfect turn in a severely unstable aircraft, say, with strong anhedral (perhaps by programming it in a simulator), and you'll see what I mean.

Of course, my question has a hypothetical element for the purpose of isolating the specific phenomenon.

Uh, no: The disturbance will awaken the dihedral effect, but this will both cause a rolling motion and a yawing moment, so this will not just reduce the disturbance, but will awaken a lot of additional effects which together will make the aircraft roll and yaw in what is called a dutch roll.

@PeterKämpf, of course, there will be many effects (this is why I said 'includes'), but if we start talking about dynamic stability, it will be yet another topic. My point was just that in the real world, 'perfect' conditions don't exist without some sort of stability, which in turn involves the effects in question. It's worth keeping this in mind.

Your point about flying with strong anhedral is very valid. +1

Would dihedral have the opposite effect in a skid then?

@RyanMortensen, yes, anhedral is 'reverse dihedral' (wing tips lower than roots). In a skid, it will tend to roll out of the turn (or into the windward side). Note though that positive wing sweep works somewhat like dihedral, so it is possible to have natural stability with slightly anhedral but swept wing. Many transport aircraft have such arrangement.

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text:
Are pilots legally required to verbally declare an emergency to invoke §91.3(b)'s deviation authorization?
Related:Do you have to explain what your emergency is?
I am aware that pilots are required to declare an emergency and inform ATC of as much information as possible during IFR flight due to this rule:

§91.183    IFR communications.
Unless otherwise authorized by ATC, the
  pilot in command of each aircraft operated under IFR in controlled
  airspace must ensure that a continuous watch is maintained on the
  appropriate frequency and must report the following as soon as
  possible—
(a) The time and altitude of passing each designated reporting point,
  or the reporting points specified by ATC, except that while the
  aircraft is under radar control, only the passing of those reporting
  points specifically requested by ATC need be reported;
(b) Any unforecast weather conditions encountered; and
(c) Any other information relating to the safety of flight.

Is there any rule that requires the declaration of a VFR emergency for it to be legally valid for the purpose of rule deviation?
My assumption is no because a 2-way radio is not even required in all locations.

No, but if you make it back on the ground you better be able to explain it.

Look at it this way, they aren't going to take enforcement action while you are in the air. The very worst that could happen is you may get a couple grey escorts making obscene gestures to "go down" or "follow". If you are busy flying the plane that is priority number 1, you only have to communicate an emergency if able.

@RonBeyer, if you make it back on the ground? Don't you mean when? Take off is optional, landing is mandatory...

@FreeMan Something like that, however more like "your body will, your soul may not".

All I see in 14 CFR 91.183 is a requirement to report. I see no requirement to declare an emergency.

The short answer is no, there is no specific requirement for VFR flights to state an emergency, however you may be expected to explain yourself and your actions once you reach the ground.

My assumption is no because a 2-way radio is not even required in all
  locations.

To expand on this you don't need a radio on board for VFR flight (FAA FAR 91.205) so they would have a legally contradicting issue if they required you to announce when you don't have a radio. As mentioned by Ron in the question you linked the rule is "Aviate, Navigate, Communicate" If you are too busy with the first two that is far more important.
Im not sure where they are pulling the quotes but this NASA presentation on emergencies sums it up well.

“…an intent of 91.3 is to ensure the PIC will handle the emergency in
  a manner necessary to save lives and not be worried about regulatory
  compliance.”
“I’ve never seen a pilot violated for deviating from a
  regulation when that pilot has either declared an emergency OR has
  stipulated in ANY written response to the FAA that an emergency
  existed at the time of the deviation.”

This article has some interesting points from a lawyers perspective on the situation,

Madsen defended a case for a pilot who deviated from an assigned
  altitude due to extreme turbulence while flying a Cessna 150. The
  pilot had dropped his handheld mic, so he couldn't initially
  communicate his situation. He stabilized the airplane and was about
  600 feet off his altitude when ATC called him. At that point he was
  able to find the mic and he told the controller what had happened.
  "It's unrealistic in some situations to communicate your emergency
  before you secure the aircraft," Madsen said. "You want to control the
  aircraft first." Madsen said the case was ultimately dismissed, but he
  felt it would have been much easier to defend had the pilot declared
  his emergency.

Here we can see that ultimately the pilot was in the right and while he should have declared, there was no legal grounds that required it.
On some level you are at the mercy of the FAA with this one and its clearly a case by case basis (as is everything in aviation) and the lawyer in the above article goes on to say

Some FAA field offices take the wording very seriously. Such was the
  case with the Cessna 150 pilot who deviated from his altitude. "They
  questioned why he didn't state an emergency and call out mayday, and
  they took great issue with the fact that he didn't use the accepted
  terms," Madsen said.

Again the case was ultimately dismissed whether or not they were happy about that.

First of all, FAA regulations are not laws, they are regulations. Federal laws are made by members of Congress elected by the people of the several states. Regulations are issued by executive-appointed bureaucrats. The word "legal" or "legally" is properly used when speaking of laws, not regulations. Violations of regulations are usually what are called "civil violations," and have nothing to do with "illegality" which normally refers to criminal actions.
When Washington bureaucrats discuss regulations they do not use the words "legal" or "illegal" for the reasons stated above. They use terms such as "compliant" or "non-compliance". Non-compliance with an FAA regulation has nothing to with "legality". I know there are a lot of people who fantasize about controlling every time a pilot wipes their butt and throwing them in jail for any imagined act of willfulness, but so far that has not happened, so please do not refer to FAA regulations as though they were laws.
As for your question, pilots are required to declare an emergency only if the circumstances permit it.
A pilot can deviate from ATC instructions any time they determine that it is required for safety. Notifying ATC of a deviation is purely secondary to the primary goal of preserving the aircraft and avoiding crashing.
If a pilot deviates from an instruction in a way that is of concern to ATC, then an explanation can be demanded by certain appropriate officials at the discretion of those officials. If they find the pilot's explanation unsatisfactory or indicative of poor judgement, they can revoke the pilot's license.

While you are technically correct ("the best kind of correct!") the difference between an FAA regulation and a "law" is largely an academic one for a pilot: The United States Code (USC) delegates rulemaking authority from Congress to various agencies, and provides for civil and/or criminal penalties when people violate the regulations promulgated by those agencies under the Code of Federal Regulations (CFR). Bust a regulation under 14CFR and you may face civil (or in some cases criminal) penalties under 49USC.

Its extremely rare for a pilot to have his/her license revoked. More often than not a punishment results in a suspension. Even the pilots of NW188 that had their ATP's revoked could reapply for them after a year. I'm having a hard time coming up with an example of permanent revocation too.

If you falsify records in a way that demonstrates unusually harsh contempt for regulations you run the risk of permanent revocation.  Photoshopping your medical to extend the date, change the class, or amend restrictions are examples.

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text:
Is the RAT pump on the A320 standard fitment or is it an "option"?
Is the RAT pump standard or optional on the A320 aircraft?
What about the B737?

What RAT "pump"? The RAT is an electrical generator, not a pump, are you asking about a supplemental hydraulic system? Or the Ram Air Turbine generator?

Depending on the system or a/c RATs drive both an AC generator and a hydraulic pump.

A RAT is never an optional piece of equipment. It is only there because the design of the aircraft systems require it in order for the aircraft to meet airworthiness certification.

The B-737 does not have a RAT.

I can find examples of dual-engine failure in a 737 (e.g. TACA 110) which is a fairly desperate situation, but none where both engines and the APU have failed in a 737 (e.g. due to fuel shortage as per Gimli 767) - I guess that would mostly be fatal in a 737.

@RedGrittyBrick    It would be a really bad day, but you'd have the battery for electrical systems, and hydraulics from the engines windmilling, and manual reversion on the flight controls (no fly-by-wire). So not necessarily unsurvivable. But any time you lose both motors, things are pretty bad!

Due to a faulty battery Garuda 421 had also the APU down.

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text:
Does EASA have access to medical records outside of the EU?
When registering for the Class 1 Part-MED Medical Certificate can/will the EASA have access to your medical records that's outside of the EU?

Probably not, but they will definitely ask you to provide them.

No, EASA cannot access anything.
You will be asked to bring your medical records to the exams. Neither EASA nor your examining doctor is able to access any of your medical records without your consent. It's restricted personal information.
Some countries already have medical records in digital (computerized) form, but many still rely on written data. In any case, EU institutions have no jurisdiction outside of EU.
You will also be given a questionnaire that will include a checkbox for any known medical conditions you have (or had in the past). I don't know what your condition is/was, but it might be a good idea to consult a private examiner first before checking anything on the list that might prevent you from getting a certificate issued.

exactly what i was looking for thanks


text:
Is turning off engines before landing "Normal"?
I was traveling by Airbus A380 recently and I was sitting near the wing.
Throughout the flight, I could hear the airplane noise even with headphones on.
Suddenly before landing, everything went silent. There was no noise at all.  There was a strange silence throughout the aircraft.  Everyone was scared and looking at each other, some even started praying.
But we landed perfectly fine. It was like routine landing.
So, I want to ask, whether it was normal? Or the pilot avoided telling us something went wrong?
P.S. Before starting the landing process, the pilot informed about the weather being fine and said "there's a safety manual in your seat pocket."

I'm pretty sure the engines were just idling. Probably other ambient noise such as gears and flaps were louder than the idling engines.

No, no pilot would want to shut off engines before landing unless it was an emergency landing. Its possible the engines went to flight-idle which may be very quiet to somebody listening in the cabin.

It was first time I had such experience. Should read about Engine Idling.

Can anyone explain that in detail

How do you think the aircraft got to the gate after landing if the engines were off?

Were thrust reversers applied after touchdown?

@Masse Also, the amount of ambient noise coming just from air rushing over the airframe shouldn't be underestimated. It's likely that the cabin was actually still relatively loud at the time, but just seemed quiet to passengers who have grown accustomed to the drone of the engines at cruise power for the last several hours.

One thing to keep in mind is that the captain and flight attendant reaction is a much better barometer of severity than the passengers.  Some passengers will believe they're in a near-death experience if one engine stalls or the turbulence is bad, but the pilot doesn't say much because he sees something like this regularly and knows the risk is minor.

There's virtually no chance of the pilot deliberately switching off all 4 engines, and even less chance of all 4 engines failing at the same time. It's not entirely impossible for it to happen, but it definitely wouldn't be quietly ignored within the aviation community. I promise you that you would've heard about it afterwards.
That said, the A380's engines are modern, quiet, and actually mounted quite a long way out on the wings, this makes them fairly quiet when idling, even when compared to other aircraft. This is particularly noticeable after a long flight where they've been running at fairly high power for 8 or more hours, and is even more the case if you're not used to the A380, eg if you usually fly on common twins such as the A320, 737, 777 which are much louder within the cabin.
I suspect, but can't be sure, that the engines were simply pulled back to idle (minimum power/thrust) and it sounded much quieter than you expected.
But no, the engines were almost certainly not simply turned off, that just doesn't happen. The only thing I can think of that would cause that would be a flame-out, and you'd definitely notice that!

This kind of implies that thrust reversers were never used and brakes only were used to slow the aircraft. Isn't that in itself uncommon?

Plus the fact that you would have stopped on the runway!

@BillOertell I can't speak to A380 operations, but on 747-100/200 aircraft, if we had a long runway and we didn't want to turn off until the end, it wasn't uncommon to not power up in reverse. It was a matter of the captain's preference. A five minute idle cooling time from last significant power was SOP, so if you avoided powering up in reverse, that five minute period started from when you went to idle just before touchdown rather than after you were down and near the end of the landing roll. Also, personally, I liked not assaulting the passengers' ears with all that noise.

@BillOertell. See (also): Why doesn't the A380 use its outboard thrust reversers?. Wikipedia's article says: "The A380 was initially planned without thrust reversers".

@BillOertell an A380 will sometimes only use idle reverse, as above. And I did indeed mean cabin rather than cockpit, particularly in good weather. Idle reverse still provides a fair bit of thrust

Two very good answers already. I just wanted to mention that as far as I can tell there are just two instances where a modern passenger jet lost all four engines. In both cases it was a B747 (in 1982 and in 1989), both due to volcanic ash, and they were both able to restart them eventually.
There is another report of a military aircraft (C17) that had a four-engine flameout in 2010 due to a lightning strike. They were still able to restart two.
In summary, it seems as there is not a single record of a four-engine passenger jet landing without operational engines.

I don't get what is the link between your post and the question. If there had been 2,000 aircraft that had landed safely without engines due to failures, what would that change to the possibility of voluntary landings without engines?

As mentioned above, what you probably experienced was the flight crew retarding the thrust levers on the engines to idle during the round out, typical of landing operations; the silence being the noticeable difference in sound as you had become used to cruise and/or descent thrust settings and the accompanying whine or noise the engines make when listened to from the cabin.  I am not aware of any procedures where a crew would knowingly shut down one or both engines of an revenue carrying airliner unless it was an emergency.
As a test of this, once the airplane touched down, did you hear a sound of the engines 'revving up' as the thrust reversers were deployed?  If so, the engines definitely were not shut down at round out.

This is an A380 too, so would require powering down of 4 engines. Unheard of outside an extreme emergency

Regarding the last part of the answer, regardless of whether reverse thrust was selected, if the plane taxied to the gate after landing rather than stopping on the runway, bringing out stairs to evacuate, and then being towed to a maintenance hangar, then the engines were definitely still running. :)

Keep in mind that if something goes bad during the approach, the airplane must be able to do a go-around, i.e. full power and again airborne. If you shut down the engines you would had no time (and speed) to restart them, and that would be against the flight safety.
Even if ALL the engines had stopped (it happened more than once, principally due to lack of fuel - you can search the history of the Gimli Glider), be sure you could notice it, because you would have landed in the nearest airport (and not your intended destination), and you would have lost the cabin illumination. In that cases, the electric systems supply power to the most importants systems of the airplane, and at the cabin you should notice this effect.
You can at either case be sure the pilot would not put the flight into danger.


text:
Does Airforce one carry missiles to protect itself and its passengers?
Do the VC-25 aircraft used for Air Force One carry missiles to protect itself and its passengers?
If so, can these missiles be fired at potential threats?

I edited it to specify the aircraft used rather than the call sign. I believe that’s what the OP intended.

Near as is know outside of a security clearance the answer is no, a Boeing VC-25 does not carry missiles or offensive weaponry.  During times of crisis, VC-25s are often escorted by armed jet fighters which can attack potential threats to the VC-25 if need be.

Following this source, the armament is "none".
An aircraft so big looks more like like a bomber, and large bombers in these days tend to rely on the escorting fighter aircraft to defend them. On board self-defence weapons of the bomber are not efficient, see this answer. I think VC-25 just does the same.
Other large aircraft with the potential management role, like Boeing E-3 Sentry, Lockheed EC-121 Warning Star or Boeing E-767 also appear unarmed even if they are much more battle oriented.

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text:
When escorting Air Force 1, at what safe distance must fighter jets fly?
Air Force 1 is sometimes escorted by fighter jets, either for security reasons or for show.  Given that Air Force 1 is a very large aircraft that produces an extremely large amount of jet blast and may very well blow and peel off certain flight critical parts of a fighter jet not designed for that type of air speed. It produces wake turbulence as well.
What is the safe distance at which fighter pilots fly their jets? How do they measure this?

Wouldn't that be rather similar to any other Boeing 747? I also question your assessment that an engine of a 747 "may very well suck an aircraft the size of a fighter jet" to the point that it becomes a problem. Fighters are still pretty large, and they have a pretty good amount of excess power when cruising alongside an airliner.

Note that fighter jets typically fly 8-9 m from a tanker aircraft (which is also very large) during aerial refueling.

Fighter jets are typically designed for rather high airspeeds.

@CptReynolds and high G-loads. And damage. It may be uncomfortable, but I imagine the difference between uncomfortable and dangerous to be really, really large for many fighters...

They can fly as close as they like so long as they're upwind of the wake turbulence. I can't imagine the engines giving them much grief, but again, it's a case of being in the right place.
Bear in mind that while Air Force One is indeed a large aircraft, fighter jets are by no means small and they also carry significant thrust.
Rather than formation flying which is far more tactical than just flying close, especially when escorting, take a look at interceptions. You'll see fighter pilots will get in very close indeed to intercepted aircraft. As an example see this interception (starts at 19:40):

As a bonus, and also in that video, check out air to air refueling.
In short, fighter aircraft are capable of getting within a few feet of almost any other aircraft quite safely - assuming stable flight and cooperation of course.


text:
How much fuel does the A380 use for taxiing before takeoff?
How much fuel does the A380 use for taxiing before takeoff and after landing? What does it cost at today's pricing?

Fuel costs vary by airport, country, and the airlines make deals with suppliers. You need to be more specific.  This link seems to generally cover what you ask https://aviation.stackexchange.com/questions/654/whats-the-typical-cost-and-its-breakdown-for-a-long-haul-commercial-flight?rq=1

Costs also depend on flight duration, passenger and cargo load, cabin crew numbers (salaries), airport taxes,  etc

@CrossRoads The question you linked answers this question. Possible duplicate.

You have two different questions here: the total cost of operating an A380 (probably answered here, as others have said), and the fuel required for taxiing (some information here but may not be exactly what you want). You might want to separate them.


text:
Would tubofan aircraft benefit from adjustable fan blades?
Would turbofan aircraft benefit from adjustable fan blades as do turboprops? Propeller aircraft adjust their propeller size, i.e. shorter propeller for cruising at maximum design altitudes where the air is thinner for efficiency, and a longer propeller where the air denser for increased thrust used for increasing altitude at early stages of flight. Would turbofans benefit from this when flying at maximum altitude for efficiency? If not why not?

"Propeller aircraft adjust their propeller size (...) shorter propeller for cruising at maximum design altitudes", They adjust their pitch angle, not their length. Is this what you mean? else can you provide an example of such variable blade-length propeller? There is a patent on that, but no application to aircraft propellers afaik.

I think the Rolls Royce Ultra Fan (in development) will feature a variable pitch fan.

@Cpt, NK-93 was such an engine, and was actually test flying in the 90s. In effect, it was a cross between a turboprop and turbofan.

Both turboprops and turbofans want their blades to be as long as they can be, in all circumstances: that way the largest amount of air is accelerated, and a given amount of thrust created with the highest efficiency. It never pays to reduce the bypass ratio of a subsonic turbofan.
Turbofans with adjustable blade pitch would make sense though, like the constant speed propellers that turboprop engines already have. This article from Flight Magazine 1973 gives the advantages of a high bypass variable pitch turbofan:

Inherently low noise levels because of high bypass ratio and low fan
  tip speeds.
Rapid thrust response, allowing thrust modulation for
  accurate rate of descent control on the approach and maximum use of
  reverse thrust during the landing run.
Reverse thrust available
  without the weight or maintenance penalty associated with reversers
  and spoilers.
High ratio of take-off to cruise thrust well matched to
  the needs of short-range, Stol aircraft.
Good cycle efficiency and specific fuel consumption.
Wide engine operating margins.
Provides many of the advantages of the turboprop while avoiding vibration,
  ground and fuselage clearance problems and allowing better matching
  with higher cruise speeds.

Actually there are situations where you do want to adjust the bypass ratio of the engine, but these are supersonic fighter engines.  E.g. https://en.wikipedia.org/wiki/General_Electric_YF120  You wouldn't see that used in any commercial sub-sonic plane.

Yes you're right of course, have added the word "subsonic"

Assuming, that engineers would have solved the design problems of such a system and making it light and easy to maintain, this would greatly enhance that flexibility of a fan engine and possibly also increase efficiency.
 It would give the designers one additional degree of freedom when designing an engine.

And add a lot of complexity (read points of failure.) Instead of balancing 2-5 blades, you would have 30-50 blades to adjust exactly the same.


text:
Is there possibility to Survive without brain damage at 30/35,000 feet?
I just read Mayday by Nelson Demille. I won't get into many details but the book talks about an airliner (similar to the Concorde) which operates at an altitude of ~60,000 ft (~18Km). Suddenly, due to a mistaken missile launch, there are holes on both sides of the plane. All the passengers become brain-damaged except 5 people who are trapped in lavatories at that time.
Now while this is fiction, could we go crazy due to lack of oxygen in case something goes wrong? How much oxygen would be needed to land safely and how much oxygen is usually kept on an airliner in case of emergencies.
A second and related question: in case of a fire or other incident, how quickly can a plane safely descend? I know and have experienced losing altitude the last 30 minutes or so of flight, probably lining up to the airport. In an emergency, how quickly can the pilot land? Is it 5-10 minutes or more? I am talking about descending from cruise altitude and come to a dead-stop.

I've made some edits, based on the book description at Amazon. Feel free to roll them back if you feel it changes the basic question in any way.

@aeroalias edited it bit more. The book says 60,000 ft so sticking with that. Also saw that the concode flew at 58k http://aviation.stackexchange.com/questions/16014/did-concorde-really-fly-above-55k-feet-as-mentioned-in-this-report hence sticking with the fantasy :)

You have a few different questions bundled here. See here for depressurization; here for the O2 system; and here for emergency descent. All of those questions have other relevant and interesting ones linked to them.

Those have to be some pretty well sealed lav doors to hold the pressure in long enough to keep people alive - I'm going to keep that in mind for my next explosive decompression incident!  :)

From a Wikipedia article on "Cloud Suck": On 14 February 2007 while practising for a paragliding contest in Australia, Polish-born German team pilot Ewa Wiśnierska-Cieślewicz was sucked into a cumulonimbus cloud, climbing at up to 20 m per second (4,000 feet per minute) to an altitude of 9,946 m (32,600 feet). She lost consciousness due to hypoxia, but regained consciousness after 30 minutes to an hour, and landed still covered in ice after a three and a half hour flight.

Depending on meals and gastrointestinal conditions explosive decompression is certainly a risk inside lavatories as well.

Long term, no, it is not.  Any exposure to pressure altitudes over 26,000 ft will eventually cause death from hypoxia, even with acclimation to the higher altitudes.  There is a period referred to as Time Of Useful Consciousness (ToC) associated with hypoxia.  This is the approximate time that a healthy adult human with good lung capacity can maintain cognitive function.  These values can be halved if an explosive decompression occurs, which is why there is a requirement that airliners have quick donning, pressure breathing oxygen masks available to the flight crew and can be donned and establish the flow of oxygen within 5 seconds.  It is also the reason that flight attendants tell passengers to first don their emergency O2 masks, then assist someone else next to them in the event the masks are deployed.
Once ToC lapses at a given pressure altitude, cognitive function rapidly begins to fade away.  The person becomes listless, sleepy, but often euphoric.  Loss of consciousness, coma and then finally death will ensue unless the minimum partial pressure of O2 in the lungs is re-established.
Section 3-1 of the FAA Manual of Aviation Physiology is a good reference on the subject of hypoxia.
At the altitudes that Concorde cruised at, the air pressure is so low that decompression sickness becomes more of a risk than hypoxia.  Having the water boil away in your own bloodstream is a very painful way to die, and loss of consciousness via hypoxia would almost be a relief from that torment.

Water in bloodstream won't boil even in vacuum. The diastolic blood pressure in healthy human is around 80 mmHg and at that value, boiling point is still 47°C, safely above your body temperature. …

… Your body will swell everywhere, content of your digestive tract will expand uncomfortably and above 63,000 ft, where boiling point of water at ambient pressure drops below body temperature, your eyes and mucous membranes will quickly dry out, but it's always the hypoxia that is fatal.


text:
Who is the woman that famously survived a light-plane crash in the arctic in the 1950s?
As a kid I read a book by a woman who crashed in the Canadian arctic and, along with the pilot, walked their way out of the bush. She lost some toes (and fingers?) to frostbite, but was otherwise OK.
I believe the action took place in the 1950s, but it might have been the very early 1960s. I seem to recall it being in the Yukon, and it was a light plane, not something like a DC-3.
She was a major celebrity for a while; I recall her getting a mail that was addressed simply "to that girl that crashed in the arctic".
Does anyone recall who this was?

I'm guessing that this is "Hey, I'm Alive" by Helen Klaben (the female survivor).
From one of the reviews:

[...] Flores's Howard five-seater crashed in a heavily wooded region, which no doubt saved their lives. The treetops softened the crash landing yet left the plane listing at an angle, making an uncomfortable shelter for either of them to lie down. Both Klaben and Flores were injured with various broken bones, and each had limited mobility. They were not prepared for an emergency and thus had no winter supplies or extra food. Their scant supply of food ran out after being stretched to siege-of-Leningrad proportions over ten days. I recall a dramatic scene from the movie where Sally Struthers, who played Klaben, discovers a tube of toothpaste which she shows Flores, played by Ed Asner. The two treat the toothpaste as a precious meal, which is exactly how Klaben described it.
  [...]
  Once all food ran out, including the toothpaste, the pair took to re-chewing their gum and by drinking melted snow. I am amazed that they could have survived on only water for five weeks. Flores, hearing a mysterious buzzing in the distance, was determined to find out what it was. I won't spoil the story by revealing if he found out the source of the buzzing or not, but when he returned to the crash site he and Klaben moved camp to a clearing, miles away from the trees. Klaben, who could not walk because her gangrenous toes were decaying, with bone pushing through the skin, was towed along on a toboggan made from the fuselage.

National Post Article on her story
People article
Quote from People:

When Ralph Flores and Helen Klaben were found alive on March 25, 1963 near the wreckage of his single-engine plane in the Yukon, their names became synonymous with “miracle.” For seven weeks the 42-year-old electrician and the 21-year-old Brooklyn girl had endured pain from crash injuries and temperatures 42° below zero while subsisting on little more than melted snow and a few biscuits.

  Flores was flying home to California from Fairbanks and Klaben had accepted his offer of an inexpensive ride to San Francisco. During their ordeal he lost 51 pounds. She dropped 45 and after their rescue had the toes of her right foot amputated because of frostbite. Still, they survived on guts and ingenuity.

They didn't actually "walk-out", they moved the camp to a clearing and were found. Klaben couldn't walk (due to the condition of her toes/feet).

Once again the wisdom of the internet never ceases to amaze. Thank you sir!


text:
Why aren't plane engines mounted closer to each other or closer to the fuselage to reduce extra forces on the wings?
Why aren't plane engines mounted closer to each other or closer to the fuselage to reduce extra forces on the wings?

Welcome, Mark! Please see https://aviation.stackexchange.com/questions/23375/how-do-you-choose-engine-placement-for-under-wings-engines and https://aviation.stackexchange.com/questions/1397/how-does-the-mounting-location-of-a-jet-engine-affect-aircraft-performance before asking. If the answers to both questions leave something open, refer to the existing answers so we know how to help you.

Placing the engines further out actually helps to relieve the load: in flight the wing provides lift and bends upwards, the fuselage hangs from the wings.

Also, a practical but un-ignorable requirement, if they were closer to the body, they would be closer to the ground during takeoff and taxiing, and the need a certain ground clearance. This helps avoid ground strikes, and improves performance during takeoff by providing undisturbed air to the engines. Engines often create a ground vortex in the intake which sucks up any dirt or objects from the ground. The CFM 56 engine has an oval intake because it's quite close to the ground.


text:
What does it mean for N1/N2 to float?
The logic of ECU to hold N1 with fuel flow follow the logic:
Idle N2------------N2 set ,N1 floats
min ps3 schedule -----------N1 and N2 float

So what does this mean? And does the float here with the same meaning of the float display on the UPPER ECAM?

Float just means that that value will vary depending on ambient temperature pressure etc. It is not controlled. E.g. on n2 control, if n2 goes down then the control puts in more fuel and if n2 goes up then the control puts in less. But if n1 changes (maybe because ambient temp changed ) the control does nothing. If n1 goes down then so be it.


text:
In theory which causes more parasitic drag? A large smooth wing or a small, rough exhaust port?
I'm new to aviation and I have a basic, mechanical question.  Wings produce a lot of lift-induced drag, especially at lower speeds.  But according to this chart that drops off as velocity increases.

However despite the fact that lift drag declines as speed increases the wings still represent roughly 70%(+/-) of the surface area of an aircraft and they do produce parasitic drag.
So my question is despite the enormous surface area that the wings represent do small, irregular shaped items such as exhaust ports produce more parasitic drag as a plane nears its top speed?
The pictures below are from a P-51 which has a top speed of over 400mph.  At that speed is the wing or the exhaust port the biggest drag producer?  Or is this maybe a close call?
Thanks so much for any input!

I'd suggest the wing has more drag - the exhaust pipes are right behind the 4-blade prop and all the turbulence they initially create.

The wing creates more drag due to the much larger area.
The wing is beautifully streamlined and the flow stays attached to the wing if the angle of attack remains below stall values. So friction drag is the only drag that acts upon the wing, while stubby shapes like exhaust ports have friction drag and pressure drag. This means that area for area, short stubby shapes have way more drag, as illustrated in the figure below from Prouty. But multiply with the length of the wing & stub, and the picture changes.

But the exhaust ports don't have pressure drag, because the exhaust gases are ejected backwards, in the spot where normally the separated airflow would extra drag.


text:
How should I interpret this raw SID data from the FAA?
I am trying to make sense of the SID structures from the FAA's 28 day NASR subscription files (SSD and/or STARDP, raw data in txt files), and I have two questions.
First, I notice that there are SIDs without any path: no transition FIX/NAVAIDs and no main bodies. What's the point of referencing a SID that does not contain anything else except the airport(s)? Take this SID for example (no other records exist for BELLEVUE SID):
D0075     NA N4729353W12212567RNT     BELVU3.BELVU BELLEVUE THREE
D0075     AA N4729353W12212567RNT

Second, another thing that prevents me from interpreting these SIDs is the difference between "NA" airports and "AA" airports. From the layout file which explains how to read and interpret the records, I read that "If there are airports served by the DP that are not served by any body, there will be one or more records describing these airports, with the FIX/facility type code field containing an 'NA'".
The problem is that in each SID where an 'NA' record exists, another one 'AA' record exists for the SAME airport described by the 'NA' record, so I cannot really understand if they are served by the SID's main body (if any). I am not sure how to interpret this, or the fact that SIDs like the one already mentioned (BELLEVUE) contain neither a main body nor any transition paths.
Another example is this SID; every "NA" airport seems to be also an "AA" airport:
D0161     NA N3832426W08950067BLV     CARDS1.CSX   CARDS ONE
D0161     NA N3844553W09022121STL
D0161     NA N3834133W09009183CPS
D0161     NA N3853237W09002456ALN
D0161     NA N3839436W09039074SUS
D0161     AA N3832426W08950067BLV
D0161     AA N3844553W09022121STL
D0161     AA N3834133W09009183CPS
D0161     AA N3853237W09002456ALN
D0161     AA N3839436W09039074SUS
D0161     ND N3845099W09021395CSX     CARDS1.BDF BRADFORD TRANSITION
D0161     R  N3921383W09013256SKYPE K5
D0161     R  N4004061W09003391SKUTR K5
D0161     NW N4109350W08935163BDF
D0161     ND N3845099W09021395CSX     CARDS1.LEBOY LEBOY TRANSITION
D0161     R  N3922076W09033098DEECE K5
D0161     R  N3926003W09034230LEBOY K5
D0161     ND N3845099W09021395CSX     CARDS1.NEENS NEENS TRANSITION
D0161     R  N3923033W09025555MYKEY K5
D0161     R  N4014535W09031538NEENS K5
D0161     ND N3845099W09021395CSX     CARDS1.SPI   SPINNER TRANSITION
D0161     R  N3919223W09000200TEWHY K5
D0161     NW N3950230W08940398SPI

Am I missing something here? I apologize for the lengthy post but I would really appreciate if someone could point me to the right direction.

To your first question, some SIDs exist to provide a basic set of information that will almost always be given as part of a clearance, rather than a path to fly. This allows a controller to not have to state that part of the clearance verbally, saving time and confusion.
On the Bellevue Three Departure from KRNT, the departure is as follows:

Climb heading 350°, maintain 3000, expect RADAR vectors, expect filed altitude within 5 minutes after departure.

There's no location to code in the procedure (other than that from the Lost Communications section) so while the SID exists, there's not much to code for an automated system to interpret.

Really helpful, thanks a lot for the thorough answer

I think another good procedure to help understand this is the BLUES6. It is used at some of the same airports and also requires vectors to CSX, but it's defined by the following lines:
D0106     ND N3845099W09021395CSX     BLUES6.ENL   BLUES SIX
D0106     NW N3825120W08909324ENL

So why is this one defined by the ND and NW lines? It's because all of the airports share the "body" of CSX to ENL before starting a transition. For the CARDS1, there is no body that is shared; all transitions start at CSX.
Even for procedures using the 'NA' code to define the procedure, the 'AA' code is still needed to list the airports that use the procedure. It does get repetitive, but that's the way they chose to code it.

Definitions from the layout file for what the NA and AA codes are used for:

AA     ADAPTED AIRPORT
  NA     NAVAID - AIRPORT
  ...
  FOLLOWING THE DESCRIPTION OF THE BASIC STAR ARE ONE OR MORE RECORDS
  DESCRIBING ALL AIRPORTS USING THIS STAR. THE FIX/FACILITY TYPE CODE
  FIELD WILL CONTAIN AN 'AA'.
  ...
  IF THERE ARE AIRPORTS SERVED BY THE DP THAT ARE NOT SERVED BY ANY BODY, THERE WILL BE ONE ONE OR MORE RECORDS DESCRIBING THESE AIRPORTS, WITH THE FIX/FACILITY TYPE CODE FIELD CONTAINING AN 'NA'. THE FIRST OF THESE RECORDS WILL CONTAIN THE COMPUTER CODE AND NAME OF THE BASIC DP.  THESE RECORDS WILL BE THE FIRST RECORDS FOR THE DP IF THERE ARE NO BODIES FOR THE DP, AND WILL FOLLOW THE LAST BODY DESCRIPTION IF THERE ARE ANY BODIES.

Thank you fooot for the time you spent analyzing this example, you re-assured me that I was heading in the right direction. I really wish I could mark both answers as the most helpful since both answered different parts of my original post.


text:
Is a Cessna 162 safe to fly if engine RPM drops by 160 on one magneto?
On a Cessna 162, during engine runup I switch to one mag, and the engine RPM dropped by 50, but on the other mag the RPM dropped by 160 while the POH says maximum allowable drop is 150. Is this plane safe to fly for local pattern work?  The only cause is just a "dirty" spark plug right?  I mean both mags are working at least.

There's a little unwritten rule in aviation; Don't do anything you wouldn't want to show up on an accident report - Do you really want "Pilot continued to departure in aircraft even though they were aware that it was not operating within the limits....... blah blah blah" to ever show up?

"maximum allowable drop" seems self-explanatory to me...

A 10 RPM exceedance would not stop me if the engine ran smoothly on that mag, ran smoothly at higher power on that mag, and the differential between the mags was OK. You're within the +/- tolerance of the tach itself.  I would certainly do a high power mag check, and even do another mag check during the climb at a safe altitude while at climb power. If there are mag problems they will be readily apparent at high power settings. If doing multiple trips or legs, I don't do mag checks after first flight of the day.

Never knowingly violate an aircraft limitation.  If it says 150 max drop, the aircraft is unairworthyy if it drops more.  It will void any insurance poilcy and you can lose your certificate if there is an incident or accident.

A student of mine took off once on only one mag.  The switch was in LEFT or RIGHT not BOTH.  About five minutes into the departure, the engine began to run rough.  Only switching to both mags solved the problem.

The engine is not performing within the limits specified in your POH. Unless some document gives a different limit for "just in the local traffic pattern," then by taking it up you'd be intentionally violating the manufacturer's prescribed operating limitations for the aircraft. While you might "get away with it," you'd have zero excuse if anything were to go wrong or if a Fed were to find out what you'd been doing.
POH limitations aren't optional.
If it is in fact just a dirty spark plug, it shouldn't take a mechanic long to make everything right.

A fouled plug will usually cause rough running on the single mag because you almost never have all 4 plugs fouled the same, so one cylinder will drop out first. It's usually the bottom plugs, where lead metal from the avgas forms as beads in the insulator well of the plug, eventually bridging the center electrode insulator.
A smooth running mag drop out of limits suggests something more upstream, in the mag itself, like an ignition timing issue or ignition coil problems, that sort of thing, that causes a lack of power on all cylinders. For example, the internal ignition points wear down, both from contact erosion and cam follower wear, and the wear causes the timing to drift in the retarded direction, reducing power very gradually.
Note that there is normally another limit you didn't mention with traditional mags, the differential between the two mag check values, usually 75 RPM. So borderline exceedance of the max drop like that I might let go, where it's a case of trying to parse 10 RPM on the gauge and maybe parallax error is skewing the reading you think you see, but you also have exceeded the 75 RPM differential between the mags, so the exceedance is pretty clear cut.
Normally the next step is to run the power higher, say to 2000-2200 RPM (try to do it on grass), and lean the mixture to  rich side of roughness, and let it go 15 seconds or so like that, then try the mag check at 1700 RPM again.  Often you'll find it passes if the plugs just had a bit of carbon that gets burned off.  That's a scenario your instructor should take you through, because it could make the difference between being stranded away somewhere on a cross country, and making it home with a normally running engine.


text:
What does the letter E signify for the E-3 AWACS?
I came across the E-3 AWACS aircraft with the designation E-3, I could not find what the letter E means in E-3.

Related

The E isn't exactly the short form of a word as it is a mission designation. In the US military's Tri-Service aircraft designation system, E indicates that the aircraft performs special electronic missions. Similarly, F implies the aircraft is a fighter aircraft and B indicates the aircraft performs bombing missions.
edit: here is the US military aircraft designation system

Sounds like E is the short form of "electronic"

@Josef yes, but they aren't all that way.  L is cold weather, K is tanker, etc.

@MarshallTigerus: I thought K for tankers came from "K"erosene (jet fuel)?

@Sean or just tanKer (like coLd weather).  The US military has a long history of it, though.  e.g. carriers being CV comes from them being a cruiser hull (C) using heavier than air aircraft (V).

@MarshallTigerus: And here I thought CV was an abbreviation for "(aircraft-) "C"arrier "V"essel".  Guess I learnt something new today!